In general, a thermally sensitive recording medium is prepared by the following process. That is, after a colorless or pale colored basic leuco dye and an organic color developing agent are respectively ground to fine particles, these particles are mixed together with additives such as a binder, a filler, a sensitizer and a slipping agent or others to obtain a coating, then the obtained coating is coated on a substrate such as a paper, a synthetic paper, a film or a plastic. The thermally sensitive recording medium develops color by an instant chemical reaction caused by heating with a thermal head, a hot stamp, a thermal pen or a laser beam. Usually, a thermally sensitive recording medium is broadly applied for a facsimile use, a terminal printer of a computer, an automatic bending machine for tickets and a recorder of a measuring instrument. Recently, along with the diversification or high qualification of a recording apparatus, high speed printing and high speed image formation are becoming possible, accordingly, more excellent quality is required to the recording sensitivity of thermally sensitive recording medium. Further, along with the diversification of usage, a recorded image of high quality is required at a whole range from lower density to higher density.
As a method to satisfy these requirement, the technique to improve the smoothness of the surface of thermally sensitive recording layer using a super calender or others is ordinary carried out, however, a sufficient high quality image can not be obtained. It is well known that the coating uniformity of the undercoating layer is important for the formation of a high quality image, for example, the improvement of the smoothness of the undercoating layer by using a super calendar is well known. Further, in Patent Document 1, for the purpose of providing a thermally sensitive recording medium having an excellent reproducibility of dot, the technique to accumulate the first intermediate layer and the second intermediate layer between a substrate and a thermally sensitive layer by order, and to adjust the OKEN smoothness of the first intermediate layer to 700 seconds or more and the density of the second intermediate layer to 0.1 or less is disclosed.
In the meanwhile, for the purpose of obtaining a highly sensitive thermally sensitive recording sheet, for example in Patent Document 2, there is a disclosure that two or more intermediate layers are provided between a substrate and a color developing layer and the uppermost layer contains from 0.3 wt % to 3 wt % of carboxymethyl cellulose and/or hydroxyethyl cellulose to pigment.
Patent Document 1: JPA 2000-108518 publication
Patent Document 2: JPA H4-348989 publication
However, by the method using a super calender, the porous property of the under coating layer is hurt and the adiabatic property is lost by calender pressure and the sensitivity is deteriorated. The method of accumulating plural intermediate layers is not advantageous from the view point of industrial production because the process becomes complicated. Therefore, the object of the present invention is to provide a thermally sensitive recording medium which does not cause the above mentioned problems, has excellent recording sensitivity and can obtain a recorded image of a high quality.